1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and more particularly, to a method of detecting a lifespan of a transfer roller in an electrophotographic image forming apparatus, and the electrophotographic image forming apparatus employing the method.
2. Description of the Related Art
As is generally known in the art, in an electrophotographic image forming apparatus, light is shed on a photosensitive medium to form an electrostatic latent image, a developing agent is applied to the electrostatic latent image to form a visual image, the visual image is transferred to a paper sheet, and the paper sheet with the visual image printed thereon is then output.
FIG. 1 illustrates a conventional electrophotographic image forming apparatus. As shown in FIG. 1, the conventional electrophotographic image forming apparatus 100 includes a laser scanning unit 110 to generate a laser beam, a photosensitive drum 120 on which an electrostatic latent image is formed, and a developing roller 130 to supply toner to the photosensitive drum 120 to form an electrostatic latent image. The apparatus 100 further includes a transfer roller 140 to transfer a visible image, which is the developed form of the electrostatic latent image of the photosensitive drum 120, onto a paper sheet, a charge roller 150 to charge the surface of the photosensitive drum 120 to a predetermined potential, a power supply 160 to supply a high voltage to the respective parts, a controller 170 to control the operation of the respective parts, and a discharge sensor 180 to detect whether the printed image is discharged or not.
In the conventional electrophotographic image forming apparatus 100 having the construction described above, when the laser beam generated in the laser scanning unit 110 is incident on the surface of the photosensitive drum 120, an electrostatic latent image is formed on a portion of the surface of the photosensitive drum 120. Then, toner supplied from the developing roller 130 is attached to the portion of the photosensitive drum 120 having the electrostatic latent image thereon, to form a visual image, which is then transferred onto a paper sheet by the transfer roller 140 applied with the high voltage from the power supply 160. After the transfer is completed, the toner remaining on the surface of the photosensitive drum 120 is eliminated by a cleaning blade 121.
Meanwhile, in the image forming apparatus 100 as described above, in order to transfer the negatively-charged toner clinging to the photosensitive drum 120 onto the paper sheet, the power supply 160 applies high voltage (e.g., 600 to 4200 volts) to the transfer roller 140, which is made of a conductive sponge, thereby forming a predetermined electric potential difference between the photosensitive drum 120 and the transfer roller 140. In this case, since the transfer roller 140 has a resistance increasing in proportion to the amount of time during which electricity is applied to the transfer roller 140, it is impossible to produce a quality image after the transfer roller 140 has been in use for too long. Therefore, the transfer roller 140 must be replaced after being used for a long time, and the controller 170 judges the time to replace the transfer roller 140 and displays the time for a user to see.
FIG. 2 is a flowchart illustrating a method of detecting a lifespan of the transfer roller 140 in the conventional electrophotographic image forming apparatus 100. First, when printing is initiated and a paper sheet is supplied from outside (S10), the controller 170 examines if a printed paper sheet has been discharged or not (S11). In this case, when a printed paper sheet has been discharged, one is added to an existing number n of total printed paper sheets (S12) and a new number n of total printed paper sheets is then compared with a critical number N (e.g., 100,000 or 150,000) of paper sheets which can be printed by the transfer roller 140 (S13). When n is less than or equal to N, the controller 170 determines if the apparatus will continue the printing or not (S15). According to this determination, the controller 170 issues a command to feed paper or a command to stop the printing.
In contrast, when n is greater than N, the controller 170 controls a display unit (not shown) disposed on a body of the image forming apparatus 100 to indicate that it is time to replace the transfer roller 140 (S14).
However, the conventional electrophotographic image forming apparatus 100, in which the lifespan of the transfer roller 140 is judged by the number n of the total printed paper sheets, cannot determine the exact lifespan of the transfer roller 140, because it does not factor the variation in the resistance of the transfer roller 140 according to the number of print jobs, which may change according to printing conditions. In other words, when a continuous printing operation lasts for a long time, resistance of the transfer roller 140 rapidly increases. As a result, even before the number n of the total printed paper sheets reaches the critical number N, the increased resistance of the transfer roller 140 may cause the printing quality to deteriorate. In contrast, when printing is intermittently carried out, there may be only a small increase in the resistance of the transfer roller 140 even after the number n of the total printed paper sheets reaches the critical number N, thereby enabling a good printing quality to be obtained.
As a result, in the conventional electrophotographic image forming apparatus 100 as described above, even when the lifespan of the transfer roller 140 expires and the transfer roller 140 should be replaced, the time to replace the transfer roller 140 may have already passed, causing the printing quality to deteriorate. On the other hand, if the lifespan of the transfer roller 140 has not yet expired, an unnecessary replacement increases the expense in maintaining the image forming apparatus.